FIG. 7 shows a general high pressure discharge lamp 11 (for example, an ultra high pressure mercury lamp) used as a light source for a projector or the like. The high pressure discharge lamp 11 is formed of a bulb 2 and a pair of electrode mounts included in the bulb 2. The bulb 2 is formed of a discharge space 3 and a pair of sealing portions 4 having the discharge space 3 disposed therebetween. Each of the electrode mounts is formed of an electrode 5, a metal foil 6, and a lead wire 7 welded to one another. A front end side of the electrode 5 is exposed to the discharge space 3 of the bulb 2, and part of an electrode shaft portion 5a on a base side of the electrode 5, the metal foil 6, and part of the lead wire 7 are embedded in the sealing portion 4. The discharge space 3 is filled with 0.15 mg/mm3 or more of mercury, a noble gas, and a halogen gas, and the mercury vapor pressure during its operation is 150 atmospheres or more.
Incidentally, the high pressure discharge lamp (hereafter, referred to as “lamp”), when used, is repeatedly turned on and off and has such a problem that the electrode shaft portion is bent when the lamp is turned on and off, which is caused by difference in coefficient of thermal expansion between the electrode shaft portion (tungsten) and the sealing portion (quartz glass). The mechanism of how the bending of the electrode shaft portion occurs is as follows. First, when the lamp is turned on, the electrode shaft portion expands in a radial direction and also expands toward the discharge space. Meanwhile, quartz glass of the sealing portion hardly expands compared to the electrode shaft portion since the coefficient of thermal expansion of quartz glass is far smaller than that of the electrode shaft portion. The electrode shaft portion expands while quartz glass of the sealing portion maintains its shape. This causes the electrode shaft portion to adhere to part of the sealing portion. Thereafter, when the lamp is turned off, the electrode shaft portion contracts to return to an original position. At this time, the adhering portion of the electrode shaft portion maintains this state while other portions moves away and become separated from the quartz glass (a gap is formed). In other words, contraction is restricted in the adhering portion of the electrode shaft portion while contraction is not restricted in a gap portion and, as a result, the electrode shaft portion is bent. The bending of electrode shaft portion causes problems such as misalignment of optical axis and reduction in illuminance.
To solve the problem of the bending of electrode shaft portion described above, Patent Document 1 describes the following configuration. A tapered portion which becomes narrower from a base to a front end is provided in an electrode shaft portion and contraction of the electrode shaft portion is thereby made less likely to be restricted by quartz glass of a sealing portion.
Moreover, in Patent Document 2, an inner surface of a sealing portion (quartz glass) and an outer surface of an electrode shaft portion are configured to support each other with a contact portion therebetween being made small. This prevents the inner surface of the sealing portion (quartz glass) from inhibiting expansion and contraction of the electrode shaft portion. Specifically, a contact portion with the electrode shaft portion is made small by configuring an inner surface structure of the sealing portion (quartz glass) in such a manner that a cross-section of the inner surface structure has a shape such as a triangle or includes a protruding portion.